Recently, so-called a layout scanner was invented and put to practical use in some fields. The functioning of such a scanner includes steps of individually storing picture data and letter (drawing) data obtained from respective input devices into a large capacity memory such as a disc memory beforehand, reading each of the data to display them on a monitor such as a CRT display, and performing layout work by using a device such as a digitizer under control of a CPU (central processing unit). In the abovementioned system, the letter data as well as picture data can be obtained by scanning an original letter copy by using an input scanner as disclosed by the present applicant in the Japanese Patent Applications Nos. 57-6650, 57-18184, and 57-39877.
Generally speaking, a pixel size for sampling is about 50.times.50 (.mu.m). So when a 10.times.10 (cm) picture is sampled by the above pixel size, and 8 bit data lines are provided for separation data Y, M, C, and K, for each of the colors which are formed from R, G and B color component signals through color correction, gradation correction etc., 4 M byte data capacity is required for each of the color separation Y, M, C, and K leading to a total requirement of 16 M byte data capacity. On the other hand, a usual color CRT used as a monitor for layout work has only 500.times.500 pixels which can afford to manage only 250K byte data for one color separation. Therefore to display said 10 cm picture on the full surface of the CRT screen, said 4 M byte data must be condensed into 250K byte data, the ratio of condensation being 1/16.
If said 16 M byte data are read from the disc memory and subjected to a condensing procedure under control of the CPU operation, comparably long time is needed for said reading and condensing procedure of the data.
In addition to that, as a condensing procedure, a method of picking up data representing one out of several pixels (in the aforesaid example, data corresponding to one pixel out of 4.times.4=16 pixels) is adopted. A resulting displayed image on the monitor is of low quality, and there is required much time if this defect is corrected by introducing a method of averaging out several data groups, which is impractical.